1. Field of the Invention
The present invention relates to a color conversion film. More specifically, the color conversion film of the present invention is a color conversion film providing excellent conversion efficiency. The present invention also relates to a multicolor emitting organic electroluminescent device comprising this color conversion film. An organic electroluminescent device is also referred to as an Organic Light-emitting Diode (OLED). Such a multicolor emitting organic electroluminescent device can be applied to personal computers, word processors, televisions, fax machines, audio and video equipment, car navigation systems, desktop calculators, telephones, portable terminals, industrial instruments and the like.
2. Description of the Related Art
In recent years, there has been active research into practical applications for organic electroluminescent elements, which are essential components of organic electroluminescent devices. Because organic electroluminescent elements can provide high current densities at low voltages, they are expected to provide a high level of brightness and luminescent efficiency, and should be applicable in particular to organic multicolor luminescent devices capable of high definition multicolor or full color display.
In terms of specific full color systems, the current focus of attention is on color conversion (CCM) systems, which combine a color conversion film with a color filter.
In a CCM system, blue or blue-green light from an organic electroluminescent layer for example is absorbed by a fluorescent dye, converting this light to visible light at longer wavelengths from green to red. From the standpoint of productivity and cost, this system is better than a separate-color RGB system in which organic electroluminescent layer materials are designed for red (R), green (G) and blue (B) pixels. From the standpoint of color reproducibility, this system is better than a white/color filter system, which combines color filters with an organic electroluminescent element emitting white light.
In a conventional CCM system using a color conversion film comprising a dye dispersed in a binder resin, the conversion efficiency is not very high when obtaining red luminescence from blue or blue-green luminescence, and red color conversion films in particular are in need of improvement.
When the concentration of the color conversion substance is increased in the color conversion film, the absorbed energy is expended as it moves back and forth between identical molecules without producing luminescence, a phenomenon that is called concentration quenching.
One means of controlling concentration quenching would be to reduce the concentration of the color conversion substance in the binder resin, but adequate converted light strength might not be obtained in this case due to reduced absorption of the light to be absorbed.
Therefore, as a means of obtaining adequate converted light strength while controlling concentration quenching, the thickness of the color conversion film is increased to thereby increase light absorption and maintain color conversion efficiency.
When such a thick color conversion film (about 10 μm thick) is used, however, the electrode pattern is interrupted by changes in level, and it may be difficult to attain high definition. When such a thick color conversion film is combined with an organic electroluminescent element, moreover, the organic electroluminescence layer may be degraded by moisture or solvent remaining in the color conversion film, resulting in display defects.
The color conversion film should also be as thin as possible because the view angle dependence is reduced if the film is too thick.
Under these circumstances, the following techniques have been disclosed in the context of color conversion films and related technologies.
Japanese Patent Application Laid-open No. 2008-056797 discloses a fluorescent material containing a polyimide having a repeating unit represented by a specific formula, and discloses in particular a fluorescent material containing a green light emitting polyimide and a purple light emitting polyimide.
Japanese Patent Application Laid-open No. 2007-157550 discloses a color conversion film with a thickness of 2 μm or less containing a first dye and a second dye, wherein the first dye absorbs light incident on the color conversion film and transfers the energy of the light to the second dye, while the second dye receives the energy from the first dye and emits light. The first dye is contained in the color conversion membrane in an amount capable of sufficiently absorbing the incident light, and the second dye is contained in the amount of not more than 10 mol % based on the total constituent molecules of the color conversion film.
With the technology described in Japanese Patent Application Laid-open No. 2008-056797, however, because the fluorescent light emission wavelength is controlled in the blue to green range by changing the copolymerization ratio of the two polyimides, excellent conversion efficiency is not achieved because the emitted light is merely the sum of the fluorescence of the individual polyimides.
In the technology described in Japanese Patent Application Laid-open No. 2007-157550, on the other hand, a thin color conversion layer with a thickness of 2 or less is achieved by means of a method using a color conversion layer containing a host dye and a guest dye, in which the host dye is excited and the guest dye produces light by energy transfer from the host dye. Because the host dye and guest dye are formed by vapor deposition in this method, there is no wet process and no problems from moisture or solvent. Because patterning is by mask deposition, however, there is room for further improvement for application to large-scale displays.
Consequently, it is an object of the present invention to provide a color conversion film which can be made thin because no binder resin is used, which is applicable to large-scale displays, and which can provide excellent conversion efficiency.
It is also an object of the present invention to provide a multicolor emitting organic electroluminescent display using such a color conversion film.